MPLS J. Le Roux, Ed.
Internet-Draft T. Morin, Ed.
Intended status: Historic France Telecom - Orange
Expires: November 27, 2011 May 26, 2011
Requirements for Point-To-Multipoint Extensions to the LabelDistribution Protocoldraft-ietf-mpls-mp-ldp-reqs-08
Abstract
This document lists a set of functional requirements that served as
input to the design of Label Distribution Protocol (LDP) extensions
for setting up point-to-multipoint (P2MP) Label Switched Paths (LSP),
in order to deliver point-to-multipoint applications over a Multi
Protocol Label Switching (MPLS) infrastructure.
Status of this Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
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and may be updated, replaced, or obsoleted by other documents at any
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material or to cite them other than as "work in progress."
This Internet-Draft will expire on November 27, 2011.
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described in the Simplified BSD License.
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Internet-Draft Reqs for P2MP extensions to LDP May 20111.3. Definitions1.3.1. Acronyms
P2P: Point-To-Point
MP2P: Multipoint-to-Point
P2MP: Point-To-MultiPoint
MP2MP: MultiPoint-To-Multipoint
LSP: Label Switched Path
LSR: Label Switching Router
PE: Provider Edge router
P: Provider router
IGP: Interior Gateway Protocol
AS: Autonomous System
1.3.2. Terminology
The reader is assumed to be familiar with the terminology in
[RFC3031], [RFC5036], and [RFC4026].
Ingress LSR:
Router acting as a sender of an LSP
Egress LSR:
Router acting as a receiver of an LSP
P2P LSP:
An LSP that has one unique Ingress LSR and one unique Egress LSR
MP2P LSP:
An LSP that has one or more Ingress LSRs and one unique Egress LSR
P2MP LSP:
An LSP that has one unique Ingress LSR and one or more Egress LSRs
MP2MP LSP:
An LSP that as one or more Leaf LSRs acting indifferently as
Ingress or Egress LSR
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Leaf LSR:
An Egress LSR of a P2MP LSP or an Ingress/Egress LSR of a MP2MP
LSP
Transit LSR:
An LSR of a P2MP or MP2MP LSP that has one or more Downstream LSRs
Branch LSR:
An LSR of a P2MP or MP2MP LSP that has more than one downstream
LSR
Bud LSR:
An LSR of a P2MP or MP2MP LSP that is an egress but also has one
or more directly connected downstream LSR(s)
P2MP tree:
The ordered set of LSRs and links that comprise the path of a P2MP
LSP from its ingress LSR to all of its egress LSRs.
1.4. Context and Motivations
LDP [RFC5036] has been deployed for setting up point-to-point (P2P)
and multipoint-to-point (MP2P) LSPs, in order to offer point-to-point
services in MPLS backbones.
There are emerging requirements for supporting delivery of point-to-
multipoint applications in MPLS backbones, such as those defined in
[RFC4834] and [RFC5501].
For various reasons, including consistency with P2P applications, and
taking full advantages of MPLS network infrastructure, it would be
highly desirable to use MPLS LSPs for the delivery of multicast
traffic. This could be implemented by setting up a group of P2P or
MP2P LSPs, but such an approach may be inefficient since it would
result in data replication at the ingress LSR and duplicate data
traffic within the network. Hence new mechanisms are required that
would allow traffic from an Ingress LSR to be efficiently delivered
to a number of Egress LSRs in an MPLS backbone on a point-to-
multipoint LSP (P2MP LSP), avoiding duplicate copies of a packet on a
given link and with MPLS traffic replication at some Branch LSRs.
RSVP-TE extensions for setting up Point-To-Multipoint Traffic
Engineered LSPs (P2MP TE LSPs), have been defined in [RFC4875]. They
meet requirements expressed in [RFC4461]. This approach is useful,
in network environments where P2MP Traffic Engineering capabilities
are needed (Optimization, QoS, Fast recovery).
However for operators who want to support point-to-multipoint traffic
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delivery on an MPLS backbone, without Traffic Engineering needs, and
who have already deployed LDP for P2P traffic, an interesting and
useful approach would be to rely on LDP extensions in order to setup
point-to-multipoint (P2MP) LSPs. This would bring consistency with
P2P MPLS applications and would ease the delivery of point-to-
multipoint services in an MPLS backbone.
1.5. Document Scope
This document focuses on the LDP approach for setting up P2MP LSPs.
It lists a detailed set of requirements for P2MP extensions to LDP,
so as to deliver P2MP traffic over a LDP-enabled MPLS infrastructure.
The original intent was that these requirements should be used as
guidelines when specifying LDP extensions.
Note that generic requirements for P2MP extensions to MPLS are out of
the scope of this document. Rather this document describes solution
specific requirements related to LDP extensions in order to set up
P2MP LSPs.
Note also that other mechanisms could be used for setting up P2MP
LSPs, such as for instance PIM extensions, but these are out of the
scope of this document. The objective is not to compare these
mechanisms but rather to focus on the requirements for an LDP
extension approach.
2. Requirements Overview
The P2MP LDP mechanism MUST support setting up P2MP LSPs, i.e. LSPs
with one Ingress LSR and one or more Egress LSRs, with traffic
replication at some Branch LSRs.
The P2MP LDP mechanism MUST allow the addition or removal of leaves
associated with a P2MP LSP.
The P2MP LDP mechanism MUST co-exist with current LDP mechanisms and
inherit its capability sets from [RFC5036]. It is of paramount
importance that the P2MP LDP mechanism MUST NOT impede the operation
of existing P2P/MP2P LDP LSPs. Also the P2MP LDP mechanism MUST co-
exist with P2P and P2MP RSVP-TE mechanisms [RFC3209], [RFC4875]. It
is of paramount importance that the P2MP LDP mechanism MUST NOT
impede the operation of existing P2P and P2MP RSVP-TE LSPs.
The P2MP LDP mechanism MAY also allow setting up multipoint-to-
multipoint (MP2MP) LSPs connecting a group of Leaf LSRs acting
indifferently as Ingress LSR or Egress LSR. This may allow a
reduction in the amount of LDP state that needs to be maintained by a
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LSR.
3. Application Scenario
Figure 1 below illustrates an LDP enabled MPLS provider network, used
to carry both unicast and multicast traffic of VPN customers
following for instance the architecture defined in
[I-D.ietf-l3vpn-2547bis-mcast] for BGP/MPLS VPNs, or the one defined
in [I-D.ietf-l2vpn-vpls-mcast].
In this example, a set of MP2P LDP LSPs are setup between Provider
Edge (PE) routers to carry unicast VPN traffic within the MPLS
backbone.
And in this example a set of P2MP LDP LSPs are setup between PE
routers acting as Ingress LSRs and PE routers acting as Egress LSRs,
so as to support multicast VPN traffic delivery within the MPLS
backbone.
For instance, a P2MP LDP LSP is setup between Ingress LSR PE1 and
Egress LSRs PE2, PE3, and PE4. It is used to transport multicast
traffic from PE1 to PE2, PE3 and PE4. P1 is a Branch LSR, it
replicates MPLS traffic sent by PE1 to P2, P3 and PE2. P2 and P3 are
non-branch transit LSRs, they forward MPLS traffic sent by P1 to PE3
and PE4 respectively.
PE1
*| *** P2MP LDP LSP
*|*****
P1-----PE2
*/ \*
*/ \*
*****/ \******
PE3----P2 P3----PE4
| |
| |
| |
PE5 PE6
Figure 1: P2MP LSP from PE1 to PE2, PE3, PE4.
If later there are new receivers attached to PE5 and PE6, then PE5
and PE6 join the P2MP LDP LSP. P2 and P3 become Branch LSRs and
replicate traffic received from P1, to PE3 and PE5, and to PE4 and
PE6 respectively (see Figure 2 below).
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PE1
*| *** P2MP LDP LSP
*|*****
P1-----PE2
*/ \*
*/ \*
*****/ \******
PE3----P2 P3----PE4
*| |*
*| |*
*| |*
PE5 PE6
Figure 2: Attachment of PE5 and PE6.
The above example is provided for the sake of illustration. Note
that P2MP LSPs ingress and egress LSRs may not necessarily be PE
routers. Also branch LSRs may not necessarily be P routers.
4. Detailed Requirements4.1. P2MP LSPs
The P2MP LDP mechanism MUST support setting up P2MP LSPs. Data plane
aspects related to P2MP LSPs are those already defined in [RFC4461].
That is, a P2MP LSP has one Ingress LSR and one or more Egress LSRs.
Traffic sent by the Ingress LSR is received by all Egress LSRs. The
specific aspects related to P2MP LSPs is the action required at a
Branch LSR, where data replication occurs. Incoming labelled data is
appropriately replicated to several outgoing interfaces which may use
different labels.
An LSR SHOULD NOT send more than one copy of a packet on any given
link of a P2MP LSP. Exceptions to this are mentioned in Section 4.9
and Section 4.18.
A P2MP LSP MUST be identified by a constant and unique identifier
within the whole LDP domain, whatever the number of leaves, which may
vary dynamically. This identifier will be used so as to add/remove
leaves to/from the P2MP tree.
4.2. P2MP LSP FEC
As with P2P MPLS technology [RFC5036], traffic MUST be classified
into a FEC in this P2MP extension. All packets which belong to a
particular P2MP FEC and which travel from a particular node MUST use
the same P2MP LSP.
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If existing FECs cannot be used for this purpose, a new LDP FEC that
is suitable for P2MP forwarding MUST be specified.
4.3. P2MP LDP Routing
As with P2P and MP2P LDP LSPs, the P2MP LDP mechanism MUST support
hop-by-hop LSP routing. P2MP LDP-based routing SHOULD rely upon the
information maintained in LSR Routing Information Bases (RIB).
It is RECOMMENDED that the P2MP LSP routing rely upon the unicast
route to the Ingress LSR to build a reverse path tree.
4.4. Setting Up, Tearing Down and Modifying P2MP LSPs
The P2MP LDP mechanism MUST support the establishment, maintenance
and teardown of P2MP LSPs in a scalable manner. This MUST include
both the existence of a large amount of P2MP LSPs within a single
network and a large amount of leaf LSRs for a single P2MP LSP (see
also Section 4.17 for scalability considerations and figures).
In order to scale well with a large number of leaves it is
RECOMMENDED to follow a leaf-initiated P2MP LSP setup approach. For
that purpose, leaves will have to be aware of the P2MP LSP
identifier. The ways a Leaf LSR discovers P2MP LSPs identifiers rely
on the applications that will use P2MP LSPs, and are out of the scope
of this document.
The P2MP LDP mechanism MUST allow the dynamic addition and removal of
leaves to and from a P2MP LSP, without any restriction (provided
there is network connectivity). It is RECOMMENDED that these
operations be leaf-initiated. These operations MUST NOT impact the
data transfer (packet loss, duplication, delay) towards other leaves.
It is RECOMMENDED that these operations do not cause any additional
processing except on the path from the added/removed Leaf LSR to the
Branch LSR.
4.5. Label Advertisement
The P2MP LDP mechanism MUST support downstream unsolicited label
advertisement mode. This is well suited to a leaf-initiated approach
and is consistent with P2P/MP2P LDP operations.
Other advertisement modes MAY also be supported.
4.6. Data Duplication
Data duplication refers to the receipt of multiple copies of a packet
by any leaf. Although this may be a marginal situation, it may also
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be detrimental for certain applications. Hence, data duplication
SHOULD as much as possible be avoided, and limited to (hopefully
rare) transitory conditions.
Note, in particular, that data duplication might occur if P2MP LSP
rerouting is being performed (See also Section 4.8).
4.7. Detecting and Avoiding Loops
The P2MP LDP extension MUST have a mechanism to detect routing loops.
This MAY rely on extensions to the LDP Loop detection mechanism
defined in [RFC5036]. A loop detection mechanism MAY require
recording the set of LSRs traversed on the P2MP Tree. The P2MP loop
avoidance mechanism MUST NOT impact the scalability of the P2MP LDP
solution.
The P2MP LDP mechanism SHOULD have a mechanism to avoid routing loops
in the data plane even during transient events.
Furthermore, the P2MP LDP mechanism MUST avoid routing loops in the
data plane, that may trigger unexpected non-localized exponential
growth of traffic.
4.8. P2MP LSP Re-routing
The P2MP LDP mechanism MUST support the rerouting of a P2MP LSP in
the following cases:
o Network failure (link or node);
o A better path exists (e.g. new link, metric change);
o Planned maintenance.
Given that P2MP LDP routing should rely on the RIB, the achievement
of the following requirements relies on the underlying routing
protocols (IGP, etc.).
4.8.1. Rerouting upon Network Failure
The P2MP LDP mechanism MUST allow for rerouting of a P2MP LSP in case
of link or node failure(s), by relying upon update of the routes in
the RIB. The rerouting time SHOULD be minimized as much as possible
so as to reduce traffic disruption.
A mechanism MUST be defined to prevent constant P2MP LSP teardown and
rebuild which may be caused by the instability of a specific link/
node in the network. This can rely on IGP dampening but may be
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completed by specific dampening at the LDP level.
4.8.2. Rerouting on a Better Path
The P2MP LDP mechanism MUST allow for rerouting of a P2MP LSP in case
a better path is created in the network, for instance as a result of
a metric change, a link repair, or the addition of links or nodes.
This will rely on update of the routes in the RIB.
4.8.3. Rerouting upon Planned Maintenance
The P2MP LDP mechanism MUST support planned maintenance operations.
It MUST be possible to reroute a P2MP LSP before a link/node is
deactivated for maintenance purposes. Traffic disruption and data
duplication SHOULD be minimized as much as possible during such
planned maintenance. P2MP LSP rerouting upon planned maintenance MAY
rely on a make before break procedure.
4.9. Support for Multi-Access Networks
The P2MP LDP mechanism SHOULD provide a way for a Branch LSR to send
a single copy of the data onto an interface to a multi-access network
(e.g. an Ethernet LAN) and reach multiple adjacent downstream nodes.
This requires that the same label be negotiated with all downstream
LSRs for the LSP.
When there are several candidate upstream LSRs on an interface to a
multi-access LAN, the P2MP LDP mechanism SHOULD provide a way for all
downstream LSRs of a given P2MP LSP to select the same upstream LSR,
so as to avoid traffic replication. In addition, the P2MP LDP
mechanism SHOULD allow for an efficient balancing of a set of P2MP
LSPs among a set of candidate upstream LSRs on a LAN interface.
4.10. Support for Encapsulation in P2P and P2MP TE Tunnels
The P2MP LDP mechanism MUST support nesting P2MP LSPs into P2P and
P2MP TE tunnels.
The P2MP LDP mechanism MUST provide a way for a Branch LSR of a P2MP
LSP, which is also a Head End LSR of a P2MP TE tunnel, to send a
single copy of the data onto the tunnel and reach all downstream LSRs
on the P2MP LSP, which are also Egress LSRs of the tunnel. As with
LAN interfaces, this requires that the same label be negotiated with
all downstream LSRs of the P2MP LDP LSP.
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Labels for P2MP LSPs and P2P/MP2P LSPs MAY be assigned from shared or
dedicated label spaces.
Note that dedicated label spaces will require the establishment of
separate P2P and P2MP LDP sessions.
4.12. IPv4/IPv6 Support
The P2MP LDP mechanism MUST support the establishment of LDP sessions
over both IPv4 and IPv6 control planes.
4.13. Multi-Area/AS LSPs
The P2MP LDP mechanism MUST support the establishment of multi-area
P2MP LSPs, i.e. LSPs whose leaves do not all reside in the same IGP
area as the Ingress LSR. This SHOULD be possible without requiring
the advertisement of Ingress LSRs' addresses across IGP areas.
The P2MP LDP mechanism MUST also support the establishment of
inter-AS P2MP LSPs, i.e. LSPs whose leaves do not all reside in the
same AS as the Ingress LSR. This SHOULD be possible without
requiring the advertisement of Ingress LSRs' addresses across ASes.
4.14. OAM
LDP management tools ([RFC3815], etc.) will have to be enhanced to
support P2MP LDP extensions. This may yield a new MIB module, which
may possibly be inherited from the LDP MIB.
Built-in diagnostic tools MUST be defined to check the connectivity,
trace the path and ensure fast detection of data plane failures on
P2MP LDP LSPs.
Further and precise requirements and mechanisms for P2MP MPLS OAM
purpose are out of the scope of this document and are addressed in
[RFC4687].
4.15. Graceful Restart and Fault Recovery
LDP Graceful Restart mechanisms [RFC3478] and Fault Recovery
mechanisms [RFC3479] SHOULD be enhanced to support P2MP LDP LSPs.
4.16. Robustness
A solution MUST be designed to re-establish connectivity for P2MP and
MP2MP LSPs in the event of failures, provided there exists network
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connectivity between ingress and egress nodes (i.e. designed without
introducing single points of failure).
4.17. Scalability
Scalability is a key requirement for the P2MP LDP mechanism. It MUST
be designed to scale well with an increase in the number of any of
the following:
o number of Leaf LSRs per P2MP LSP;
o number of Downstream LSRs per Branch LSR;
o number of P2MP LSPs per LSR.
In order to scale well with an increase in the number of leaves, it
is RECOMMENDED that the size of a P2MP LSP state on a LSR, for one
particular LSP, depend only on the number of adjacent LSRs on the
LSP.
4.17.1. Orders of Magnitude Expected in Operational Networks
Typical orders of magnitude that we expect should be supported are:
o tens of thousands of P2MP trees spread out across core network
routers;
o hundreds, or a few thousands, of leaves per tree;
See also section 4.2 of [RFC4834].
4.18. Backward Compatibility
In order to allow for a smooth migration, the P2MP LDP mechanism
SHOULD offer as much backward compatibility as possible. In
particular, the solution SHOULD allow the setup of a P2MP LSP along
non-Branch Transit LSRs that do not support P2MP LDP extensions.
Also, the P2MP LDP solution MUST co-exist with current LDP mechanisms
and inherit its capability sets from [RFC5036]. The P2MP LDP
solution MUST NOT impede the operation of P2P/MP2P LSPs. A P2MP LDP
solution MUST be designed in such a way that it allows P2P/MP2P and
P2MP LSPs to be signalled on the same interface.
5. Shared Trees
For traffic delivery between a group of N LSRs that act as egress
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and/or egress nodes on different P2MP flows, it may be useful to
setup a shared tree connecting all these LSRs, instead of having N
P2MP LSPs. This would reduce the amount of control and forwarding
state that has to be maintained on a given LSR.
There are actually two main options for supporting such shared trees:
o This could rely on the applications protocols that use LDP LSPs.
A shared tree could consist of the combination of a MP2P LDP LSP
from Leafs LSRs to a given root node, with a P2MP LSP from this
root to Leaf LSRs. For instance with Multicast L3 VPN
applications, it would be possible to build a shared tree by
combining (see [I-D.ietf-l3vpn-2547bis-mcast]):
* a MP2P unicast LDP LSP, from each PE of the group to a
particular root PE acting as tree root,
* with a P2MP LDP LSP from this root PE to each PE of the group.
o Or this could rely on a specific LDP mechanism allowing to setup
multipoint-to-multipoint MPLS LSPs (MP2MP LSPs).
The former approach (Combination of MP2P and P2MP LSPs at the
application level) is out of the scope of this document while the
latter (MP2MP LSPs) belong to the scope of this document.
Requirements for the set up of MP2MP LSPs are listed below.
5.1. Requirements for MP2MP LSPs
A Multipoint-to-multipoint (MP2MP) LSP is an LSP connecting a group
of Leaf LSRs acting as Egress and/or Ingress LSRs. Traffic sent by
any Leaf LSR is received by all other Leaf LSRs of the group.
Procedures for setting up MP2MP LSPs with LDP SHOULD be specified.
An implementation that support P2MP LDP LSPs MAY also support MP2MP
LDP LSP.
The MP2MP LDP procedures MUST NOT impede the operations of P2MP LSP.
Requirements for P2MP LSPs, set forth in Section 4, apply equally to
MP2MP LSPs. Particular attention should be given on the below
requirements:
o The solution MUST support recovery upon link and transit node
failure and be designed to re-establish connectivity for MP2MP
LSPs in the event of failures, provided there exists network
connectivity between ingress and egress nodes (i.e. designed
without introducing single points of failure);
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o The size of MP2MP state on a LSR, for one particular MP2MP LSP,
SHOULD only depend on the number of adjacent LSRs on the LSP;
o Furthermore, the MP2MP LDP mechanism MUST avoid routing loops that
may trigger exponential growth of traffic. Note that this
requirement is more challenging with MP2MP LSPs as a LSR may need
to receive traffic for a given LSP on multiple interfaces.
There are additional requirements specific to MP2MP LSPs:
o It is RECOMMENDED that a MP2MP MPLS LSP is built based on the
unicast route to a specific LSR called root LSR;
o It is RECOMMENDED to define several root LSRs (e.g. a primary and
a backup) to ensure redundancy upon root LSR failure;
o The receiver SHOULD NOT receive back a packet it has sent on the
MP2MP LSP;
o The solution SHOULD avoid that all traffic between any pair of
leaves is traversing a root LSR (similarly to PIM-Bidir trees) and
SHOULD provide the operator with means to minimize the delay
between two leaves;
o It MUST be possible to check connectivity of a MP2MP LSP in both
directions.
6. Evaluation Criteria6.1. Performance
The solution will be evaluated with respect to the following
criteria:
(1) Efficiency of network resource usage;
(2) Time to add or remove a Leaf LSR;
(3) Time to repair a P2MP LSP in case of link or node failure;
(4) Scalability (state size, number of messages, message size).
Particularly the P2MP LDP mechanism SHOULD be designed with as key
objective to minimize the additional amount of state and additional
processing required in the network.
Also, the P2MP LDP mechanism SHOULD be designed so that convergence
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times in case of link or node failure are minimized, in order to
limit traffic disruption.
6.2. Complexity and Risks
The proposed solution SHOULD NOT introduce complexity to the current
LDP operations to such a degree that it would affect the stability
and diminish the benefits of deploying such solution.
7. Security Considerations
It is expected that addressing the requirements defined in this
document should not introduce any new security issue beyond those
inherent to LDP, and that a P2MP LDP solution will rely on the
security mechanisms defined in [RFC5036] (e.g. TCP MD5 Signature).
An evaluation of the security features for MPLS networks may be found
in [RFC5920], and where issues or further work is identified by that
document, new security features or procedures for the MPLS protocols
will need to be developed.
8. IANA Considerations
This informational document makes no requests to IANA for action.
9. Acknowledgments
We would like to thank Christian Jacquenet, Hitoshi Fukuda, Ina
Minei, Dean Cheng, and Benjamin Niven-Jenkins, for their highly
useful comments and suggestions. We would also like to thank Adrian
Farrel for reviewing this document before publication.
We would also like to thank authors of [RFC4461] from which some of
the text in this document has been inspired.
10. References10.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3031] Rosen, E., Viswanathan, A., and R. Callon, "Multiprotocol
Label Switching Architecture", RFC 3031, January 2001.
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